Steel product and method of producing the product

ABSTRACT

A method of producing a steel product includes heat treating a mechanically worked low carbon, medium carbon or high strength low alloy steel product and maintaining or increasing the ductility and maintaining or increasing the yield stress of the steel.

The present invention relates to a steel product for use in the mining,construction and general manufacturing industries.

The present invention also relates to a method of producing the steelproduct.

The steel may be any one of low carbon steel, medium carbon steel andhigh strength low alloy steel (which is also described in the steelindustry as a microalloy steel).

The term “low carbon steel” is understood herein to mean steel havingless than 0.3 wt. % C, other elements such as Si and Mn that are addedas deliberate additions to the steel, residual/incidental impurities andbalance Fe.

The term “medium carbon steel” is understood herein mean steel having0.3-2.0 wt. % C, other elements such as Si and Mn that are added asdeliberate additions to the steel, residual/incidental impurities, andbalance Fe.

The term “residual/incidental impurities” covers elements such as Cu,Sn, Mo, Al, Zn, Ni, and Cr that may be present in very smallconcentrations, not as a consequence of specific additions of theseelements but as a consequence of standard steel making practices. Forexample, the elements may be present as a consequence of the use ofscrap steel to produce high strength low alloy, low carbon and mediumcarbon steels.

The term “high strength low alloy steel” is understood herein to meansteel of the following typical composition, in wt. %:

C: 0.07-0.30;

Si: 0.9 or less;Mn: 2.0 or less;Mo: 0.35 or less;Ti: 0.1 or less;V: 0.1 or less;Nb: 0.1 or less;Cu: 0.1 or less;N: 0.02 or less;S: 0.05 or less;Al: 0.05 or less;Residual/incidental impurities: 1.0 or less; andFe: balance.

The term “residual/incidental impurities” in the context of highstrength low alloy steels is understood as described above in relationto low and medium carbon steels. The concentrations of elements such asCu and Mo in the table in the preceding paragraph are totalconcentrations, i.e. the concentrations of these elements as a total ofdeliberate additions and residual/incidental impurities.

The steel product may be any suitable product.

The steel product may be wire, rod, bar, or strip.

The steel product may be in the form of a steel product that is madefrom any one of wire, rod, bar, and strip.

The steel product may include any product, including but not limited toreinforcement bar for concrete construction, reinforcement mesh for theconcrete construction and mining industries made by welding togetherspaced-apart parallel line wires and spaced-apart parallel cross-wires,pipe made from steel strip, couplers for coupling together any elongateproducts such as reinforcing bars, continuous spirals, ligatures forreinforcing cages for concrete columns and beams, fasteners (includingscrews, bolts etc.) made from steel bar, rock bolts made from steel bar,and other steel products used in tensile or compression or shear orflexural applications in the concrete construction, construction, miningor manufacturing industries.

The present invention is based on a surprising finding that it ispossible to treat steel by heating steel (hereinafter referred to as“heat treatment”) that has been mechanically worked (e.g. cold formedsuch as by cold rolling) and: (a) maintain or increase the ductility(for example, measured as elongation and described in the specificationin terms of elongation and known by the term Agt (uniform elongation)when referring to reinforcing steels and often expressed asAgt_((−0.5%))), (b) maintain or increase the yield stress (YS) (oftenexpressed as Proof Stress (PS) for reinforcing steels) and (c) maintainor increase the tensile strength (TS) of the steel. This is a surprisingfinding because metallurgy teaches that heat treatment ofmechanically-worked steel results in an increase in ductility and adecrease in the yield stress and a decrease in the tensile strength ofthe steel.

By way of example, the applicant found that steel that had beenmechanically worked to reduce the transverse cross-sectional area of thesteel by 5-30% and in some instances up to 75% and then heat treated ata temperature in a range of 150-750° C. for a time period of 1 minute to16 hours maintained and in many instances produced an increase inductility of at least 25% relative to that of the mechanically workedsteel and an increase in yield stress of at least 5% relative to that ofthe mechanically worked steel.

In general terms, the applicant found that mechanically worked steelcould be heat treated at higher temperatures and for shorter times or atlower temperatures and for longer times to maintain or increaseductility, yield stress, and tensile strength.

It is noted that the invention is not confined to mechanical workingthat changes the transverse cross-sectional area of a feed steel or asteel product and also extends to situations where cold working changesthe shape of the feed steel or the steel product.

Typically, and without limiting the scope of the present invention,specific steel chemistries and process routes and properties aresummarized in the following table.

Steel Process HT Temp and YS (PS) - Elongation Chemistry Route Cold WorkTime MPa (Agt) % HSLA Cold work Less than 150-750° C. Greater Greaterand HT 20%- and 5 mins- than 600 MPa than 1.5% could be 16 hrs up to ormore than 35% Low C Cold work 20-25%- 150-750° C. Greater Greater and HTcould be and 5 mins- than 500 MPa than 1.5% up to or 16 hrs more than45% Medium C Cold work 20-75% 150-750° C. 750-1000 MPa Greater and HTand 5 mins- than 1.5% 16 hrs Nb - “HT” referred to in the above Tablemeans “heat treatment”.

The present invention is based on an extensive research and developmentprogram that has focused on testing a substantial number of samples oflow carbon steels, medium carbon steels, and high strength low alloysteels. The samples included samples that were mechanically worked underdifferent conditions and heat treated at different temperatures and fordifferent times. The research and development program is discussed in alater section of the specification in more detail.

The present invention provides a method of producing a steel productthat includes heat treating a mechanically worked steel product andmaintaining or increasing the ductility and maintaining or increasingthe yield stress of the steel.

The present invention also provides a method of producing a steelproduct that includes heat treating a mechanically worked steel productand maintaining or increasing the ductility and maintaining orincreasing the yield stress and maintaining or increasing the tensilestrength of the steel.

The present invention also includes a mechanically worked and heattreated steel product. The steel product may be any one of the steelproducts described above, i.e. wire, rod, bar, or strip, and any steelproduct that is made from any one of wire, rod, bar, and strip andincluding the specific products mentioned above.

The invention provides an opportunity to use the same starting material,such as a high strength low alloy steel, low carbon steel, and mediumcarbon steel, and produce a range of required mechanical properties byappropriate selection of mechanical working and heat treatment time andheat treatment temperature.

In this regard, the present invention also provides a method ofproducing a steel product that includes selecting a feed steel as astarting material for the product and selecting mechanical working andheat treatment time and heat treatment temperature conditions of thefeed steel or a product made from the feed steel to provide requiredmechanical properties for the product and carrying out mechanicalworking and heat treating steps and maintaining or increasing theductility and maintaining or increasing the yield stress of the steeland producing the product with the required mechanical properties.

The invention provides an opportunity for small or large quantities ofreadily available steel materials to be used to manufacture:

(a) high strength (e.g. >750 MPa yield stress) and high ductility (e.g.Uniform Elongation >1.5% Agt), bar, rod, wire or mesh; and(b) medium strength (e.g. >500 MPa yield stress) and high ductility(e.g. Uniform Elongation >1.5% Agt) bar, rod wire or mesh.

By way of example, a 750 MPa yield stress type (a) steel in accordancewith the invention represents a potential material saving of 33% for thesame performance as a conventional 500 MPa yield stress reinforcingsteel in tensile applications. Therefore, the diameter of a reinforcingsteel could be reduced from, say, 12 mm to approximately 9.8 mm for thesame performance. Alternatively, using a bar of 12 mm in diameter andwith a 750 MPa yield stress would allow an increase in performance of50%, and hence e.g. a better performing concrete column or beam for thesame quantity of steel. A mesh manufactured from the material with thesame properties and used in mining applications represents a potentialmaterial saving of at least 30% for the same performance and withconsequent occupational health and safety benefits, i.e. handling of alighter product. Whilst not critical, being able to also increase theductility is a potential benefit.

By way of further example, in the concrete construction industry, beingable to manufacture 500 MPa mesh with >5% Agt allows approximately a 20%reduction in the amount of steel required in applications that requiremoment redistribution, e.g. many suspended floors. Steel fixing inAustralia is currently charged at a $/tonne rate, and therefore areduction in the amount of steel to be fixed provides an opportunity tosignificantly reduce the installed cost of reinforcing. This samereduction would apply to high strength bar or wire reinforcing.

By way of further example, using a high tensile strength (650 MPa orgreater yield stress), ductile mesh manufactured in this manner wouldpotentially allow in the order of 20-25% reduction in the mass of steelrequired to reinforce a concrete slab on ground or tilt-up concreteproducts, for example.

Each of these above-described high tensile strength or medium tensilestrength products have an added advantage of providing an opportunity tosignificantly reduce embodied energy (greenhouse gases) in the productand the potential to reduce concrete use in columns and beams andassociated reductions in transport and other materials handling costs.

Elongation is a measure of ductility. Elongation is expressed herein asUniform Elongation—Agt. The term “Uniform Elongation” is understoodherein to be a measure of the ability of steel to deform bothelastically and plastically before reaching its maximum tensilestrength. The numerical amounts for elongation reported in thespecification are the elongation of steel in percentage terms measuredafter the maximum tensile strength of the steel has been reached anddropped to 99.5% of the maximum tensile strength and expressed asA_(gt(−0.5%)). This method is used for reliability of measurement. Totalelongation is also used as a measure of ductility of steel products,particularly sheet.

The increase in elongation of the heat treated steel relative to that ofthe mechanically worked steel may be greater than 5%.

The increase in elongation of the heat treated steel may be greater than10%.

The increase in elongation of the heat treated steel may be greater than15%.

The increase in elongation of the heat treated steel may be greater than20%.

The increase in elongation of the heat treated steel may be greater than30%.

The increase in elongation of the heat treated steel may be greater than50%.

The increase in elongation of the heat treated steel may be greater than100%.

The increase in elongation of the heat treated steel may be greater than150%.

The increase in elongation of the heat treated steel may be greater than200%.

The increase in the yield stress of the heat treated steel relative tothat of the mechanically worked steel may be greater than 5%.

The increase in the yield stress of the heat treated steel may begreater than 10%.

The increase in the yield stress of the heat treated steel may begreater than 15%.

The increase in the yield stress of the heat treated steel may begreater than 20%.

The increase in the yield stress of the heat treated steel may begreater than 30%.

The increase in the yield stress of the heat treated steel may begreater than 40%.

The heat treatment step may be carried out at any suitable temperature.There are a number of factors that may have an impact on the selectionof the heat treatment temperature in any given situation. One factor isheat treatment time. The applicant has also found that each heattreatment temperature has a time window within which the yield stressand ductility are increased to a level above a desired minimum. Thiswindow narrows as the heat treatment temperature increases. Anotherfactor is the steel composition. Another factor is the targetproperties, such as ductility and yield stress.

The heat treatment step may be carried out at a temperature below theaustenitising temperature of the steel. It is noted that in any givensituation the actual temperature of the steel during the heat treatmentwill be a time-temperature dependent relationship and a function of thesteel composition. Therefore, the temperature of the furnace may beabove the austenitising temperature of the steel.

The heat treatment step may be carried out at a temperature below 1000°C.

The heat treatment step may be carried out at a temperature below 800°C.

The heat treatment step may be carried out at a temperature below 750°C.

The heat treatment step may be carried out at a temperature below 700°C.

The heat treatment step may be carried out at a temperature below 600°C.

The heat treatment step may be carried out at a temperature below 550°C.

The heat treatment step may be carried out at a temperature below 500°C.

The heat treatment step may be carried out at a temperature below 450°C.

The heat treatment step may be carried out at a temperature below 400°C.

The heat treatment step may be carried out at a temperature below 300°C.

The heat treatment step may be carried out at a temperature below 250°C.

The heat treatment step may be carried out at a temperature above 200°C.

The heat treatment step may be carried out at a temperature above 150°C.

The heat treatment step may be carried out at a temperature above theaustenitising temperature of the steel provided the heat treatment timeis selected to be sufficiently short to maintain or increase the yieldstress and maintain or increase the ductility relative to the startingpoints for yield stress and tensile strength and ductility.

The heat treatment step may be carried out for any suitable time. Thereare a number of factors that may have an impact on the selection of theheat treatment time. As discussed above in relation to heat treatmenttemperature, these factors include heat treatment temperature and steelcomposition and target properties and productivity.

The heat treatment step may be carried out for less than 16 hours.

The heat treatment step may be carried out for less than 10 hours.

The heat treatment step may be carried out for less than 6 hours.

The heat treatment step may be carried out for less than 5 hours.

The heat treatment step may be carried out for less than 4 hours.

The heat treatment step may be carried out for greater than 1 hour.

The heat treatment step may be carried out for greater than 45 minutes.

The heat treatment step may be carried out for greater than 30 minutes.

The heat treatment step may be carried out for greater than 10 minutes.

The heat treatment step may be carried out for greater than 5 minutes.

The heat treatment step may be carried out for greater than 1 minute.

The heat treatment step may be carried out for greater than 30 seconds.

The heat treatment step may be carried out in any suitable atmosphere.The atmosphere may be an oxidising atmosphere or a reducing atmosphere.By way of particular example, the heat treatment step may be carried outin air.

The heat treatment step may be carried out without a protectiveatmosphere. This is an important advantage of the invention.

The heat treatment step may be carried out using any suitable means.Specifically, any suitable source of heat energy may be used to carryout the heat treatment.

The mechanically worked steel product may be any suitable form ofproduct. The mechanically worked steel product may be in the form of anyone of wire, rod, bar, or strip.

The steel product may be in the form of any one of wire, rod, bar, orstrip.

The rod and bar products may range from products which have small tolarge aspect ratios of length to diameter. In other words, the rod andbar products may range from products which have diameters that are closeto the length of the products to products that have diameters ortransverse cross-sectional areas that are significantly less than thelength of the products.

The steel product may be in the form of a steel product that is madefrom any one of wire, rod, bar, and strip. A non-exclusive range ofsteel products is set out above. One particular steel product ofinterest to the applicant is reinforcement mesh for the concreteconstruction and mining industries made by welding together spaced-apartparallel line wires and spaced-apart parallel cross-wires. Anotherparticular steel product of interest to the applicant is reinforcing barof all kinds, such as in straight lengths and formed into ligatures orcontinuous spirals or other commonly available shapes (noting that thereare many such shapes). The invention and the properties achieved by theinvention are not limited by the shape of the steel product.

The mechanically worked steel product may be a cold rolled or drawn orany other suitable mechanically worked product that results in a changeof cross-sectional shape of the product, without necessarily changingthe transverse cross-sectional area, such that there has been energyinput required to cause the shape change. For example, the shape changemay be from a circular to an oval transverse cross-section of the samecross-sectional area as the circular shape.

The mechanically worked steel product may be a cold rolled or drawn orany other suitable mechanically worked product that has a reducedtransverse cross-sectional area after it has been mechanically worked.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 2% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 5% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 10% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 15% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 20% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 40% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 50% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 60% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 70% less than the transversecross-sectional area of the steel product before the mechanical working.

The method may include cooling the heat treated product from the heattreatment temperature at any suitable cooling rate. For example, theheat treated product may be quenched by being water-cooled. By way offurther example, the heat treated product may be cooled in ambient air.The applicant has found that, in general, the cooling rate does not havea significant impact on properties, namely ductility, yield stress andtensile strength. However, the applicant has found that quenching theheat treated product may have a significant impact on the properties insome situations, such as when quenching from heat treatment temperaturesof at least 750° C. after a particular time. In one example, afterapproximately 8 minutes at 750° C. there was a sudden increase intensile strength and a reduction in yield stress and A_(gt). Thisresponse is typical of a steel that is heat treated at a temperatureabove the austenitising temperature. In this example, there was a heattreatment window of up to 8 minutes for which subsequent quenching hadno impact on properties.

The steel may be a low carbon steel, as described above.

The steel may be a medium carbon steel, as described above.

The steel may be a high strength low alloy steel, as defined above.

The high strength low alloy steel may contain greater than 0.040 wt. %V.

The high strength low alloy steel may contain greater than 0.050 wt. %V.

The high strength low alloy steel may contain greater than 0.060 wt. %V.

The high strength low alloy steel may contain greater than 0.005 wt. %N.

The high strength low alloy steel may contain greater than 0.015 wt. %N.

The high strength low alloy steel may contain greater than 0.018 wt. %N.

The high strength low alloy steel may contain other alloying elements,such as Nb.

The present invention provides a method of producing a steel productthat includes:

-   -   (a) mechanically working a feed steel,    -   (b) heat treating the mechanically worked feed steel and        maintaining or increasing the ductility and maintaining or        increasing the yield stress of the steel; and    -   (c) forming a steel product.

The method may include multiple sequences of steps (a) and (b) and (c).

The present invention provides a method of producing a steel productthat includes:

-   -   (a) mechanically working a steel product, and    -   (b) heat treating the mechanically steel product and maintaining        or increasing the ductility and maintaining or increasing the        yield stress of the steel.

The method may include multiple sequences of steps (a) and (b).

The present invention provides a method of producing a steel productthat includes:

-   -   (a) mechanically working a feed steel,    -   (b) forming the steel product, and    -   (c) heat treating the steel product and increasing or        maintaining the ductility and maintaining or increasing the        yield stress of the steel product.

The method may include multiple sequences of steps (a) and (b) and (c).

The present invention provides a method of producing a steel productthat includes:

-   -   (a) mechanically working a feed steel,    -   (b) forming the steel product, and    -   (c) heat treating the formed steel product and maintaining or        increasing the ductility and maintaining or increasing the yield        stress and tensile strength of the steel product.

The method may include multiple sequences of steps (a) and (b) and (c).

The increase in elongation of the heat treated steel may be greater than5% relative to that of the mechanically worked feed steel.

The increase in elongation of the heat treated steel may be greater than10%.

The increase in elongation of the heat treated steel may be greater than20%.

The increase in elongation of the heat treated steel may be greater than30%.

The increase in elongation of the heat treated steel may be greater than50%.

The increase in elongation of the heat treated steel may be greater than100%.

The increase in elongation of the heat treated steel may be greater than150%.

The increase in elongation of the heat treated steel may be greater than200%.

The increase in the yield stress of the heat treated steel may begreater than 10%.

The increase in the yield stress of the heat treated steel may begreater than 20%.

The increase in the yield stress of the heat treated steel may begreater than 30%.

The increase in the yield stress of the heat treated steel may begreater than 40%.

The method may also include forming the steel product into another steelproduct.

The feed steel may be any one of low carbon steel, medium carbon steel,and high strength low alloy steel.

The feed steel may be in any suitable form. The feed steel may be in theform of any one of wire, rod, bar, or strip.

It is noted that the mechanical working step may comprise reducing thetransverse cross-sectional area, i.e. the diameter, of wire, rod andbar.

It is also noted that the mechanical working step may comprise reducingthe transverse cross-sectional area, i.e. the thickness, of the strip.

It is also noted that the mechanical working step may result in a changeof cross-sectional shape of the product, without necessarily changingthe transverse cross-sectional area, such that there has been energyinput required to cause the shape change.

The steel product may be any suitable form of product.

The steel product may be in the form of a steel product that is madefrom any one of wire, rod, bar, and strip.

The mechanical working step (a) may include cold rolling or drawing orany other suitable mechanical working step that reduces the transversecross-sectional area of the feed steel.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 2% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 5% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 10% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 15% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 20% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 40% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 50% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 60% less than the transversecross-sectional area of the steel product before the mechanical working.

The reduced transverse cross-sectional area of the mechanically workedsteel product may be at least 70% less than the transversecross-sectional area of the steel product before the mechanical working.

The heat treatment step may be carried out at a temperature below theaustenitising temperature of the steel.

The heat treatment step may be carried out at a temperature below 1000°C.

The heat treatment step may be carried out at a temperature below 800°C.

The heat treatment step may be carried out at a temperature below 750°C.

The heat treatment step may be carried out at a temperature below 700°C.

The heat treatment step may be carried out at a temperature below 600°C.

The heat treatment step may be carried out at a temperature below 550°C.

The heat treatment step may be carried out at a temperature below 500°C.

The heat treatment step may be carried out at a temperature below 450°C.

The heat treatment step may be carried out at a temperature below 400°C.

The heat treatment step may be carried out at a temperature below 300°C.

The heat treatment step may be carried out at a temperature below 250°C.

The heat treatment step may be carried out at a temperature above 200°C.

The heat treatment step may be carried out at a temperature above 150°C.

The heat treatment step may be carried out for less than 16 hours.

The heat treatment step may be carried out for less than 10 hours.

The heat treatment step may be carried out for less than 6 hours.

The heat treatment step may be carried out for less than 5 hours.

The heat treatment step may be carried out for less than 4 hours.

The heat treatment step may be carried out for greater than 1 hour.

The heat treatment step may be carried out for greater than 45 minutes.

The heat treatment step may be carried out for greater than 30 minutes.

The heat treatment step may be carried out for greater than 10 minutes.

The heat treatment step may be carried out for greater than 5 minutes.

The heat treatment step may be carried out for greater than 1 minute.

The heat treatment step may be carried out for greater than 30 seconds.

The heat treatment step (b) may be carried out in any suitableatmosphere.

The present invention also provides a steel product made by the abovemethod.

The steel product may have a yield stress of at least 500 MPa yieldstress and a Uniform Elongation of at least 1.5% Agt.

The present invention also provides a mechanically worked and heattreated high strength low alloy steel product that has a steelcomposition, an elongation and a yield stress as described above.

The steel product may have a tensile strength as described.

The present invention also provides a mechanically worked and heattreated low carbon steel product that has a steel composition, anelongation and a yield stress as described above.

The steel product may have a tensile strength as described.

The present invention also provides a mechanically worked and heattreated medium carbon steel product that has a steel composition, anelongation and a yield stress as described above.

The steel product may have a tensile strength as described.

The steel product may be in the form of a steel product that is madefrom any one of wire, rod, bar, and strip as described above.

By way of particular example, the steel product is a mesh product thatincludes parallel line wires and parallel cross-wires welded together atthe intersections of the wires, with the wires being steel wires, withthe wires being at least 3 mm in diameter, and with the wires havingbeen mechanically worked and heat treated prior to being welded togetherto form the mesh, such that the wires have a yield stress of at least650 MPa and a Uniform Elongation of at least 1.5% Agt.

By way of further particular example, the steel product is a meshproduct that includes parallel line wires and parallel cross-wireswelded together at the intersections of the wires, with the wires beingat least 3 mm in diameter, with the wires being steel wires, with thewires having been mechanically worked prior to being welded together toform the mesh, and with the mesh being heat treated, such that the wireshaving a yield stress of at least 650 MPa and a Uniform Elongation of atleast 1.5% Agt.

By way of particular example, the steel product is a ligature formedfrom a steel wire that is at least 3 mm in diameter, and with the wirehaving been mechanically worked and heat treated prior to being formedinto the ligature, such that the wires have a yield stress of at least650 MPa and a Uniform Elongation of at least 1.5% Agt.

By way of particular example, the steel product is a ligature formedfrom a steel wire that is at least 3 mm in diameter, and with the wirehaving been mechanically worked prior to being formed into the ligature,with the ligature being heated treated, such that the wires have a yieldstress of at least 650 MPa and a Uniform Elongation of at least 1.5%Agt.

The present invention is described further with reference to theaccompanying FIGS. 1-33 which are graphs of different combinations ofyield stress (Proof Stress—MPa), tensile strength (MPa), elongation(measured as Uniform Elongation—A_(gt)), and heat treatment time for lowcarbon steel, medium carbon steel and high strength low alloy samplestreated in accordance with the invention.

The present invention is based on an extensive research and developmentprogram that focused on testing a substantial number of samples of lowcarbon steel, medium carbon steel and high strength low alloy. Thesamples included samples mechanically worked under different conditionsand heat treated at different temperatures and for different times. Akey finding of the research and development program was that mechanicalworking of the steel samples was critical to maintaining or obtainingimprovements in elongation in subsequent heat treatment of the samplesand also obtaining improvements in or maintaining yield stress andtensile strength in subsequent heat treatment of the samples.

The research and development program was carried out on steel wiresuitable for use in the manufacture of reinforcing mesh and otherreinforcement products for the mining and construction industries. Thesteel wire was made from low carbon steel, medium carbon steel, and highstrength low alloy steel. The steel wire was made by rolling a largerdiameter steel rod or wire to smaller diameters.

The following is a summary of the research and development program inrelation to low carbon steel, medium carbon steel, and high strength lowalloy steel.

-   -   Steel compositions—High strength low alloy steel, low carbon        steel and medium carbon steel. Examples of the steel        compositions are set out below.

High Strength Low Alloy C Mn Si P S Cu Ni Cr Mo V Al Nb Ti CE .17 1.10.2 .013 .040 .28 .07 .11 .01 .102 .002 .001 .001 .42 .18 1.06 .25 .014.046 .28 .07 .10 .01 .093 .002 .001 .001 .42 Low Carbon C P Mn Si S NiCr Mo Cu Al-T B .06 .006 .50 .15 .009 .006 .012 .001 .014 .002 .0003 .18.010 .71 .20 .012 .005 .001 .008 .001 .0003 Medium Carbon C P Mn Si S NiCr Mo Cu Al-T B .31 .018 .70 .24 .012 .002 .010 .001 .004 .001 .0003

-   -   Initial rod product—conventional AS 1442 or similar rolling        procedure in a rod mill to produce a range of different diameter        rod samples—the rod samples were then cold rolled to smaller        diameter wires to form test samples. The samples included (a) 10        mm diameter rod rolled to 9.5 mm wire, (b) 8 mm diameter rod        rolled to 7.7 mm, 7.6 mm, 7.5 and 6.75 mm wire, (c) 10.5 mm rod        rolled to 9.5 mm, (d) 8.5 mm rod rolled to 6.75 mm, (e) 12 mm        diameter rod rolled to 10.7 mm wire, (f) 8.5 mm diameter rod        rolled to 7.6 mm wire, (g) 5.5 mm diameter rod rolled to 4.75 mm        wire with was subsequently straightened, (h) 5.5 mm diameter rod        rolled to 4.75 mm wire which was subsequently straightened using        a smaller diameter straightening roll than the straightening        roll used for the item (h) samples, and (i) 5.5 mm diameter rod        rolled to 3.06 mm wire.    -   Heat treatment furnace—a fan forced air furnace and a resistance        heated furnace.    -   Heat treatment temperatures—see Figures.    -   Heat treatment times—see Figures.    -   Air cool for samples having test data reported in FIGS. 1-21 and        26-33 and water quench for samples having test data reported in        FIGS. 22-25.    -   Sample size—approximately 300 mm long    -   Testing procedures—tensile tests on Instron machine and        elongation determined via an extensometer. The results in the        Figures include graphs of proof stress (PS), with yield stress        reported as Proof stress, elongation reported as Uniform        Elongation (A_(gt(−0.5%))), and tensile strength (TS).

The results of the research work are summarised in part in FIGS. 1-33 ofthe specification which are described and discussed below. It is notedthat FIGS. 1-25 focus on work on high strength low alloy steel (“HSLA”)samples, FIGS. 26-32 focus on low carbon steel samples, and FIG. 33focuses on medium carbon steel samples.

FIG. 1 is a graph of elongation (Agt) versus heat treatment time (0-30minutes) for HSLA samples heat treated at 300, 400, 500, 600, and 700°C., with the samples comprising 9.5 mm diameter wire samples cold rolledfrom 10 mm diameter rod in accordance with the invention. It is evidentfrom FIG. 1 that the ductility of the samples increased with heattreatment time at each heat treatment temperature, with the rate ofincrease in ductility increasing with heat treatment temperature.

FIG. 2 is a graph of yield stress (Proof Strength—MPa) versus heattreatment time (0-30 minutes) for HSLA samples heat treated at 300, 400,500, 600, and 700° C., with the samples comprising 9.5 mm diameter wiresamples cold rolled from 10 mm diameter rod in accordance with theinvention. It is evident from FIG. 2 that there was an increase in yieldstress of the cold drawn samples at short heat treatment times at eachof the heat treatment temperatures. The yield stress of the samples heattreated at the higher temperatures decreased (e.g. 500, 600, and 700°C.) as the heat treatment times increased. However, there was nodecrease in yield stress with heat treatment time for the samples thatwere heat treated at lower temperatures (300 and 400° C.). The increasein yield stress was achieved with relatively short heat treatment timesacross the range of heat treatment temperatures. This is potentiallysignificant in terms of processing times and costs.

FIG. 3 is a graph of tensile strength (MPa) versus heat treatment time(0-30 minutes) for samples heat treated at 300, 400, 500, 600, and 700°C., with the samples comprising 9.5 mm diameter HSLA wire samplesmechanically worked by being cold rolled from 10 mm diameter rod inaccordance with the invention. The cold rolling amounts to a 9.75%reduction in transverse cross-sectional area. It is evident from FIG. 3that there was an increase in tensile strength of the cold rolledsamples at short (less than 4 minutes) heat treatment times at each ofthe heat treatment temperatures. The tensile strength of the samplesthat were heat treated at the higher temperatures (e.g. 500, 600, and700° C.) decreased as the heat treatment times increased. However, therewas no decrease in tensile strength with heat treatment time for thesamples that were heat treated at lower temperatures (300 and 400° C.).In addition, the increase in tensile strength was achieved withrelatively short heat treatment times across the range of heat treatmenttemperatures. This is potentially significant in terms of processingtimes and costs.

FIG. 4 includes a graph of elongation (measured as UniformElongation—Agt) versus heat treatment time (0-5 hours) for 9.5 mmdiameter HSLA wire samples cold rolled from 10 mm diameter rod and heattreated at 300° C. in accordance with the invention. The cold rollingamounts to a 9.75% reduction in transverse cross-sectional area. Thisgraph is described as the “N10PLUS” curve in the Figure. FIG. 4 alsoincludes comparative data for 6.75 mm diameter low carbon steel wiresamples cold rolled from 8.5 mm diameter rod and heat treated in thesame way. The cold rolling amounts to a 37% reduction in transversecross-sectional area. This graph is described as the “6.75EX8.5” curvein FIG. 4. FIG. 4 illustrates increases in ductility that might beexpected as a consequence of the heat treatment of either steel samples.

FIG. 5 includes graphs of yield stress (reported as Proof Stress—MPa)versus heat treatment time (0-5 hours) for the FIG. 4 samples (HSLA andlow carbon steel) heat treated at 300° C.

FIG. 6 includes graphs of tensile strength (MPa) versus heat treatmenttime (0-5 hours) for the FIG. 4 samples (HSLA and low carbon steel) heattreated at 300° C.

It is evident from FIGS. 4-6 that there was an increase in each ofductility, yield stress, and tensile strength of the N10PLUS HSLAsamples whereas there was the conventional response of an increase inductility and a decrease in yield stress and tensile strength of the6.75EX8.5 low carbon steel samples. An interesting point in relation tothe N10PLUS samples is that the results were achieved at a low heattreatment temperature of 300° C.

FIGS. 4, 5 and 6 include dotted line sections. The experimental workreported in these Figures was very early work and the applicant chosethe heat treatment times of 1.15, 2 and 4 hours because conventionalwisdom indicates that at least 1.2 hours would be required to produce anormal reaction for a low carbon steel, i.e. an increase in ductilityand a decrease in yield stress and tensile strength—a normal recoveryheat treatment reaction. When the applicant treated the N10PLUS samplesand realized there was a strength increase, the applicant theninvestigated the shorter heat treatment times for this material. Thisalso prompted the applicant to look at shorter heat treatment times forthe 6.75 mm material (and subsequently all others) and the applicantfound an increase for the low carbon steels at the shorter times. FIGS.26, 27 and 28 demonstrate the increase in ductility, yield stress andtensile strength. This was a different material rolled to 6.75 mm andhence the different starting strengths. FIGS. 29-32, which were lowcarbon steels treated at 500° C. for the short times showed the sameincreases in ductility, yield stress and tensile strength for short heattreatment times.

FIG. 7 is a graph of elongation (Agt) versus heat treatment temperature(0-500° C.) for HSLA samples heat treated for 4 hours, with the samplescomprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wire samples cold rolledfrom 8 mm diameter rod in accordance with the invention. The sampleswere cold rolled to different extents, with the highest reduction beingaround 12% in transverse cross-sectional area. It is evident from FIG. 7that there was an increase in ductility in the cold drawn samples atheat treatment temperature greater than 200° C. and that the ductilityincreased as the heat treatment temperature increased.

FIG. 8 is a graph of yield stress (Proof Stress MPa) versus heattreatment temperature (0-500° C.) for HSLA samples heat treated for 4hours, with the samples comprising 7.5 mm, 7.6 mm, and 7.7 mm diameterwire samples cold rolled from 8 mm diameter rod in accordance with theinvention. The samples were cold rolled to different extents, with thehighest reduction being around 12% in transverse cross-sectional area.It is evident from FIG. 8 that the yield stress of each of the colddrawn samples initially increased and then decreased as the heattreatment temperature increased. The yield stress was higher for thesamples having higher cold reductions. The shapes of the graphs in FIG.8 indicate that there is a window of heat treatment temperatures, namelya window in the range of 150-400° C., in which there was a significantincrease in yield stress of the samples. The yield stress of the sampleswas higher across the whole heat treatment temperature range than theyield stress of the samples prior to heat treatment.

FIG. 9 is a graph of tensile strength (MPa) versus heat treatmenttemperature for HSLA samples heat treated for 4 hours, with the samplescomprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wire samples cold rolledfrom 8 mm diameter rod in accordance with the invention. The sampleswere cold rolled to different extents, with the highest reduction beingaround 12% in transverse cross-sectional area. It is evident from FIG. 9that the tensile strength of each of the cold rolled samples initiallyincreased and then decreased as the heat treatment temperatureincreased. The tensile strength was higher for the samples having highercold reductions. The shapes of the graphs in FIG. 9 indicate that therewas a window of heat treatment temperatures, namely a window in therange of 150-350° C., in which there was a significant increase intensile strength of the samples.

FIG. 10 is a graph of elongation (Agt) versus heat treatment time (0-7hours) for HSLA samples heat treated at 100° C., with the samplescomprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wire samples cold rolledfrom 8 mm diameter rod. The samples were cold rolled to differentextents, with the highest reduction being around 12% in transversecross-sectional area. It is evident from FIG. 10 that there was anoverall slight decrease in ductility in the cold drawn samples acrossthe range of heat treatment times. This decrease is consistent with astrain ageing mechanism. Basically, the ductility change wasconventional and the teaching is that the heat treatment temperature of100° C. was too low. The ductility was higher for the samples havinglower cold reductions.

FIG. 11 is a graph of yield stress (Proof Strength—MPa) versus heattreatment time (0-7 hours) for HSLA samples heat treated at 100° C.,with the samples comprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wiresamples cold drawn from 8 mm diameter rod in accordance with theinvention. The samples were cold drawn to different extents, with thehighest reduction being around 12% in transverse cross-sectional area.It is evident from FIG. 11 that there was an increase (albeit notsubstantial) in yield stress in the cold drawn samples across the rangeof heat treatment times. The yield stress was higher for the sampleshaving higher cold reductions.

FIG. 12 is a graph of tensile strength (MPa) versus heat treatment time(0-7 hours) for HSLA samples heat treated at 100° C., with the samplescomprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wire samples cold rolledfrom 8 mm diameter rod in accordance with the invention. The sampleswere cold rolled to different extents, with the highest reduction beingaround 12% in transverse cross-sectional area. It is evident from FIG.12 that there was a slight change in tensile strength in the cold rolledsamples across the range of heat treatment times. The tensile strengthwas higher for the samples having higher cold reductions.

FIG. 13 is a graph of elongation (Agt) versus heat treatment time (0-16hours) for HSLA samples heat treated at 300° C., with the samplescomprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wire samples cold rolledfrom 8 mm diameter rod in accordance with the invention. The sampleswere cold rolled to different extents, with the highest reduction beingaround 12% in transverse cross-sectional area. It is evident from FIG.13 that after an initial sudden decrease in ductility (which isconsistent with normal ageing) there was a significant initial increasein ductility in a relatively short heat treatment time (up to 30minutes) at 300° C. for each of the samples and that the ductilitytended to level out after around 3 hours of heat treatment at thattemperature. The ductility was higher for the samples having lower coldreductions.

FIG. 14 is a graph of yield stress (Proof Stress—MPa) versus heattreatment time (0-16 hours) for HSLA samples heat treated at 300° C.,with the samples comprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wiresamples cold rolled from 8 mm diameter rod in accordance with theinvention. The samples were cold rolled to different extents, with thehighest reduction being around 12% in transverse cross-sectional area.It is evident from FIG. 14 that there was a significant initial increasein yield stress in a relatively short heat treatment time (0-45 minutes)at 300° C. for each of the samples and that the yield stress tended tolevel out after around 45 minutes of heat treatment at that temperature.The yield stress was higher for the samples having higher coldreductions. The yield stress of the samples was higher across the wholeheat treatment temperature range than the yield stress of the samplesprior to heat treatment.

FIG. 15 is a graph of tensile strength (MPa) versus heat treatment time(0-16 hours) for HSLA samples heat treated at 300° C., with the samplescomprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wire samples cold rolledfrom 8 mm diameter rod in accordance with the invention. The sampleswere cold rolled to different extents, with the highest reduction beingaround 12% in transverse cross-sectional area. It is evident from FIG.15 that there was a significant initial increase in tensile strength ina relatively short heat treatment time (0-45 minutes) at 300° C. foreach of the samples and that the tensile strength tended to level outafter around 45 minutes of heat treatment at that temperature. Thetensile strength was higher for the samples having higher coldreductions. The tensile strength of the samples was higher across thewhole heat treatment time range than the tensile strength of the samplesprior to heat treatment.

FIG. 16 is a graph of elongation (Agt) versus heat treatment time (0-30minutes) for HSLA samples heat treated at 300° C., with the samplescomprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wire samples cold rolledfrom 8 mm diameter rod in accordance with the invention. These sampleswere cold rolled and heat treated under the same conditions as the FIG.13 samples. The samples were cold rolled to different extents, with thehighest reduction being around 12% in transverse cross-sectional area.This graph focuses on the first 30 minutes of heat treatment timehighlighted in the discussion of FIG. 13. It is evident from FIG. 16that after an initial decrease in ductility (which is consistent withnormal ageing) there was a steady increase in ductility with heattreatment time at 300° C. for each of the samples, with the ductilitybeing higher for the samples having lower cold reductions.

FIG. 17 is a graph of yield stress (Proof Stress—MPa) versus heattreatment time (0-30 minutes) for HSLA samples heat treated at 300° C.,with the samples comprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wiresamples cold rolled from 8 mm diameter rod in accordance with theinvention. The samples were cold rolled to different extents, with thehighest reduction being around 12% in transverse cross-sectional area.These samples were cold rolled and heat treated under the sameconditions as the FIG. 14 samples. This graph focuses on the first 30minutes of heat treatment time highlighted in the discussion of FIG. 14.It is evident from FIG. 17 that there was generally a steady increase inyield stress with heat treatment time at 300° C. for each of thesamples. The yield stress was higher for the samples having higher coldreductions. The increase in yield stress is well above what would beexpected from normal strain ageing. Normal strain ageing is detrimentalbecause it leads to a reduction in ductility.

FIG. 18 is a graph of tensile strength (MPa) versus heat treatment time(0-30 minutes) for HSLA samples heat treated at 300° C., with thesamples comprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wire samples coldrolled from 8 mm diameter rod in accordance with the invention. Thesamples were cold rolled to different extents, with the highestreduction being around 12% in transverse cross-sectional area. Thesesamples were cold rolled and heat treated under the same conditions asthe FIG. 15 samples. This graph focuses on the first 30 minutes of heattreatment time highlighted in the discussion of FIG. 15. It is evidentfrom FIG. 18 that there was a steady increase in tensile strength withheat treatment time at 300° C. for each of the samples. The tensilestrength was higher for the samples having higher cold reductions.

FIG. 19 is a graph of elongation (Agt) versus heat treatment time (0-30minutes) for HSLA samples heat treated at 500° C., with the samplescomprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wire samples cold rolledfrom 8 mm diameter rod in accordance with the invention. The sampleswere cold rolled to different extents, with the highest reduction beingaround 12% in transverse cross-sectional area. It is evident from FIG.19 that after an initial decrease in ductility (which is consistent withnormal ageing) there was a steady increase in ductility with heattreatment time at 500° C. for each of the samples, with the ductilitybeing higher for the samples having lower cold reductions.

FIG. 20 is a graph of yield stress (Proof Stress—MPa) versus heattreatment time (0-30 minutes) for HSLA samples heat treated at 500° C.,with the samples comprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wiresamples cold rolled from 8 mm diameter rod in accordance with theinvention. The samples were cold rolled to different extents, with thehighest reduction being around 12% in transverse cross-sectional area.It is evident from FIG. 20 that there was an initial increase in yieldstress at the heat treatment temperature of 500° C. for each of sample,with the yield stress of each sample reaching a maximum yield stressafter 10 minutes. The yield stress of each sample decreased with heattreatment times greater than 10 minutes. The yield stress was higher forthe samples having higher cold reductions. The yield stress of thesamples was higher across the whole heat treatment temperature rangethan the yield stress of the samples prior to heat treatment.

FIG. 21 is a graph of tensile strength (MPa) versus heat treatment time(0-30 minutes) for HSLA samples heat treated at 500° C., with thesamples comprising 7.5 mm, 7.6 mm, and 7.7 mm diameter wire samples coldrolled from 8 mm diameter rod in accordance with the invention. Thesamples were cold rolled to different extents, with the highestreduction being around 12% in transverse cross-sectional area. It isevident from FIG. 21 that there was an initial increase in tensilestrength at the heat treatment temperature of 500° C. for each sample,with the tensile strength of each sample reaching a maximum tensilestrength after 10 minutes, and the tensile strength of each sampledecreasing with heat treatment times greater than 10 minutes. Thetensile strength was higher for the samples having higher coldreductions.

FIG. 22 is a graph of elongation (Agt) versus heat treatment time (0-20minutes) for 6.75 mm diameter HSLA wire samples cold rolled from 8 mmdiameter rod and heat treated at 750° C. over a time period up to 20minutes and then water quenched in accordance with the invention. Thecold rolling amounts to a 29% reduction in transverse cross-sectionalarea. It is evident from FIG. 22 that after an initial decrease inductility (which is consistent with normal strain ageing) there was asteady increase in ductility with heat treatment time at 750° C. until 7minutes, followed by a sudden decrease in ductility and a suddenincrease before levelling out at around 10-12 minutes. It is evidentfrom FIG. 22 that water quenching heat treated samples had nodetrimental impact on the ductility at heat treatment times between 2and 7 minutes. It is evident from a comparison of the results in FIG. 22and the results in FIG. 19 for 7.5, 7.6, 7.7 mm material cold rolledfrom 8 mm diameter rod and heat treated at 500° C. that the ductility ofthe 6.75 mm material of FIG. 22 was higher than for the 7.5, 7.6, 7.7 mmmaterial of FIG. 19. This finding is contrary to the evidence for the7.5, 7.6, 7.7 mm material shown in FIG. 19 where the ductility decreasedwith increasing cold reduction. This may be a consequence of the higherheat treatment temperature for the 6.75 mm material generating greaterductility.

FIG. 23 is a graph of yield stress (Proof Strength—MPa) and tensilestrength (MPa) versus heat treatment time (0-20 minutes) for 6.75 mmdiameter HSLA wire samples cold rolled from 8 mm diameter rod and heattreated at 750° C. and then water quenched in accordance with theinvention. It is evident from FIG. 23 that water quenching samples thatwere heat treated for up to 7 minutes and then quenched had animprovement in yield stress and tensile strength. Heat treatment timesless than 8 minutes followed by quenching resulted in a significantincrease in tensile strength and a significant reduction in yieldstress. It is evident from FIG. 23 that there was a heat treatment timewindow of up to 7 minutes at the heat treatment temperature in whichthere was an improvement in yield stress and tensile strength. Quenchingdoes not destroy mechanical properties. An advantage of quenching isthat the product is immediately available.

FIG. 24 is a graph of elongation (Agt) versus heat treatment time (0-20minutes) for 6.75 mm diameter HSLA wire samples cold rolled from 8 mmdiameter rod and heat treated at 500° C. and then water quenched inaccordance with the invention. It is evident from FIG. 24 that after aninitial decrease in ductility (which is consistent with normal strainageing) there was a steady increase in ductility with heat treatmenttime at 500° C. It is evident from FIG. 24 that water quenching heattreated samples had no detrimental impact on ductility at heat treatmenttimes greater than 5 minutes. In addition, it is also evident that thehigher heat treatment temperature of 750° C. for the samples referred toin the preceding paragraph generated approximately 2% greater ductilitythan for the samples heat treated at 500° C.

FIG. 25 is a graph of yield stress (Proof Strength—MPa) and tensilestrength (MPa) versus heat treatment time (0-20 minutes) for 6.75 mmdiameter HSLA wire samples cold rolled from 8 mm diameter rod and heattreated at 500° C. and then water quenched in accordance with theinvention. It is evident from FIG. 25 that water quenching heat treatedsamples had substantially no impact on yield stress and tensile stress.In other words, at this heat treatment temperature there is no downsidein water quenching treated steel. It is noted nevertheless that theseheat treatment conditions produced an increase in yield stress andtensile strength.

FIGS. 26-31 focus on the results of research and development work on lowcarbon steel samples.

FIG. 26 is a graph of elongation (Agt) versus heat treatment time (0-30minutes) for samples heat treated at 500° C., with the samplescomprising 9.5 mm and 6.75 mm diameter low carbon steel wire samplescold rolled from 10 mm diameter and 8.5 mm rod respectively inaccordance with the invention. The cold rolling amounts to a 18% and 37%reduction in transverse cross-sectional area, respectively. It isevident from FIG. 26 that after an initial decrease in ductility (whichis consistent with normal strain ageing) there was a steady increase inductility with heat treatment time at 500° C.

FIG. 27 is a graph of yield stress (Proof Strength—MPa) versus heattreatment time (0-30 minutes) for samples heat treated at 500° C., withthe samples comprising 9.5 mm and 6.75 mm diameter low carbon steel wiresamples cold rolled from 10.5 mm diameter and 8.5 mm rod respectively inaccordance with the invention. The cold rolling amounts to a 18% and 37%reduction in transverse cross-sectional area, respectively. It isevident from FIG. 27 that the yield stress of the more heavilymechanically worked sample (i.e. the 6.75 mm sample) initially increased(up to 2 minutes heat treatment time) and then decreased with heattreatment time, with a period of 7 minutes treatment time passing beforethe yield stress decreased to the initial start strength, i.e. coldworked strength. The initial increase in yield stress is a surprisingresult and indicates that there is a heat treatment window in which itis possible to achieve an increase in yield stress. It is also evidentfrom FIG. 27 that the yield stress of the less heavily mechanicallyworked sample (i.e. the 9.5 mm sample) was not adversely affected byheat treatment for up to 8 minutes. When considered in conjunction withFIG. 26, the yield stress results reported in FIG. 27 are a significantresult because the results indicate that it is possible to heat treatsuch heavily worked steel and achieve the FIG. 26 increase in ductilitywithout a loss of yield stress and more importantly with a possibleincrease in yield stress.

FIG. 28 is a graph of tensile strength (MPa) versus heat treatment time(0-30 minutes) for samples heat treated at 500° C., with the samplescomprising 9.5 mm and 6.75 mm diameter low carbon steel wire samplescold rolled from 10.5 mm diameter and 8.5 mm rod respectively inaccordance with the invention. The cold rolling amounts to a 18% and 37%reduction in transverse cross-sectional area, respectively. It isevident from FIG. 28 that the tensile strength of the less heavilymechanically worked sample (i.e. the 9.5 mm sample) initially increased(up to 8 minutes heat treatment time) and then decreased with heattreatment time. The initial increase in tensile strength is a surprisingresult and indicates that there is a heat treatment window in which itis possible to achieve an increase in tensile strength. When consideredin conjunction with FIGS. 26 and 27, the FIG. 28 result is a significantresult because it indicates that it is possible to heat treat suchheavily worked steel and achieve the FIG. 26 increase in ductility andthe FIG. 27 increase in yield stress without a loss of tensile strength.

FIG. 29 is a graph of yield stress (Proof Strength—MPa), tensilestrength (MPa), and elongation (A_(gt)) versus heat treatment time (0-15minutes) for samples heat treated at 500° C., with the samplescomprising 10.7 mm diameter low carbon steel wire samples cold rolledfrom 12 mm diameter rod in accordance with the invention. The coldrolling amounts to a 20% reduction in transverse cross-sectional area ofthe samples. The plots for each parameter are shown as lines of best fitfor the actual data points. It is evident from FIG. 29 that yieldstress, tensile strength and ductility increased steadily with heattreatment time. The increase in yield stress is a surprising result. Theyield stress of the samples was higher across the whole heat treatmenttemperature range than the yield stress of the samples prior to heattreatment.

FIG. 30 is a graph of yield stress (Proof Strength—MPa), tensilestrength (MPa), and elongation (A_(gt)) versus heat treatment time (0-15minutes) for samples heat treated at 500° C., with the samplescomprising 8.5 mm diameter low carbon steel wire samples cold rolledfrom 7.6 mm diameter rod in accordance with the invention. The coldrolling amounts to a 20% reduction in transverse cross-sectional area ofthe samples. The plots for each parameter are shown as lines of best fitfor the actual data points. It is evident from FIG. 30 that yield stressand tensile strength initially increased with heat treatment time andreached a peak around 5 minutes and then gradually decreased with longerheat treatment times. The initial increase in yield stress is asurprising result and indicates that there is a heat treatment window inwhich it is possible to achieve an increase in yield stress. The yieldstress of the samples was higher across the whole heat treatmenttemperature range than the yield stress of the samples prior to heattreatment. It is also evident from FIG. 30 that ductility increasedsteadily with heat treatment time.

FIG. 31 is a graph of yield stress (Proof Strength—MPa), tensilestrength (MPa), and elongation (A_(gt)) versus heat treatment time (0-15minutes) for samples heat treated at 500° C., with the samplescomprising 4.75 mm diameter low carbon steel wire samples cold rolledfrom 5.5 mm diameter rod in accordance with the invention. The wiresamples were passed through a straightener before being heat treated.The cold rolling amounts to a 25% reduction in transversecross-sectional area of the samples. The plots for each parameter areshown as lines of best fit for the actual data points. It is evidentfrom FIG. 31 that yield stress and tensile strength initially increasedquite quickly with heat treatment time and reached a peak around 2-3minutes and then gradually decreased with longer heat treatment times.The initial increase in yield stress is a surprising result andindicates that there is a heat treatment window in which it is possibleto achieve an increase in yield stress. The yield stress of the sampleswas higher across the whole heat treatment temperature range than theyield stress of the samples prior to heat treatment. It is also evidentfrom FIG. 31 that ductility increased steadily with heat treatment time.

FIG. 32 is a graph of yield stress (Proof Strength—MPa), tensilestrength (MPa), and elongation (A_(gt)) versus heat treatment time (0-15minutes) for samples heat treated at 500° C., with the samplescomprising 4.75 mm diameter low carbon steel wire samples cold rolledfrom 5.5 mm diameter rod in accordance with the invention. The wiresamples were passed through a straightener before being heat treated.The cold rolling amounts to a 25% reduction in transversecross-sectional area of the samples. The only difference between theexperimental procedure for this experiment and the experiment reportedin FIG. 31 relates to the type of straightener used. The straighteningroll was a smaller diameter straightening roll than the straighteningroll used for the item (h) samples. The plots for each parameter areshown as lines of best fit for the actual data points. It is evidentfrom FIG. 32 that yield stress and tensile strength initially increasedquite quickly with heat treatment time and reached a peak around 2-3minutes and then gradually decreased with longer heat treatment times.The initial increase in yield stress is a surprising result andindicates that there is a heat treatment window in which it is possibleto achieve an increase in yield stress. The yield stress of the sampleswas higher across the whole heat treatment temperature range than theyield stress of the samples prior to heat treatment. It is also evidentfrom FIG. 32 that ductility increased steadily with heat treatment time.The experimental results in FIGS. 31 and 32 are very similar, save thatthe yield stress, tensile strength and elongation are somewhat higherwith the FIG. 31 straightener than the FIG. 32 straightener.

FIG. 33 is a graph of yield stress (Proof Strength—MPa), tensilestrength (MPa), and elongation (A_(gt)) versus heat treatment time (0-15minutes) for samples heat treated at 500° C., with the samplescomprising 3.06 mm diameter medium carbon steel wire samples cold rolledfrom 5.5 mm diameter rod in accordance with the invention. The coldrolling amounts to a 69% reduction in transverse cross-sectional area,respectively, of the samples. The plots for each parameter are shown aslines of best fit for the actual data points. It is evident from FIG. 33that yield stress initially increased quite quickly with heat treatmenttime and reached a peak around 3 minutes and then gradually decreasedwith longer heat treatment times. The initial increase in yield stressis a surprising result and indicates that there is a heat treatmentwindow in which it is possible to achieve an increase in yield stress.The yield stress of the samples was higher across the whole heattreatment temperature range than the yield stress of the samples priorto heat treatment. It is also evident from FIG. 33 that ductilityincreased steadily with heat treatment time.

The experimental work carried out by the applicant indicates that thereis no difference in the way in which the invention works with ribbed andsmooth wires treated in accordance with the invention.

In general terms, as illustrated by the results of the research worksummarised in the Figures, the applicant found surprisingly that theductility (measured as elongation), the yield stress, and the tensilestrength of the wire of high strength low alloy, medium carbon, and lowcarbon steels could be increased as a consequence of a combination ofmechanical working and heat treatment. The finding is a significantfinding for the following reasons:

-   -   It is possible to significantly reduce the amount of steel        required to manufacture products without a loss of force        capacity of the steel in the products. The reduced amount of        steel required for products improves the economics of        construction and reduces the carbon footprint.    -   There is an opportunity for higher strength and ductility        products.    -   There is a possibility of changing the design and resultant cost        of composite products that are made from the steel products. One        example is steel reinforced concrete products used in the        construction industry. The invention may make it possible to        reduce the amount of steel and/or the amount of concrete used in        these products or to increase the structural performance of        these products for a given amount of steel.    -   The method is inexpensive in that it can be carried out with low        capital and operating costs.

The present invention can be used at different stages in the manufactureof end-use products and therefore provides considerable flexibility. Forexample, steel wire can be processed in accordance with the invention toincrease the yield stress and ductility of the wire and then coiled.

The coiled product can be formed into end use products such as spirals,ligatures etc. Alternatively, standard wire can be produced and coiledand then processed to produce products such as mesh sheets and ligaturesetc. and these products can be processed in accordance with theinvention to increase the yield stress and ductility of the products.

Many modifications may be made to the invention described above withoutdeparting from the spirit and scope of the invention.

By way of example, the research and development program reported abovehas focused on wire. However, the view of the applicant is that theresults are found with wire should translate to rod, bar, and stripsteel products.

1. A method of producing a steel product includes heat treating a mechanically worked low carbon, medium carbon or high strength low alloy steel product and maintaining or increasing the ductility and maintaining or increasing the yield stress of the steel.
 2. The method defined in claim 1 also includes maintaining or increasing the tensile strength of the steel.
 3. The method defined in claim 1 wherein the increase in ductility, measured as elongation, of the heat treated steel relative to that of the mechanically worked steel is greater than 5%.
 4. The method defined in claim 1 wherein the increase in the yield stress of the heat treated steel relative to that of the mechanically worked steel is greater than 5%.
 5. A method of producing a steel product includes: (a) mechanically working a low carbon, medium carbon or high strength low alloy steel feed steel, and (b) heat treating the mechanically worked feed steel and maintaining or increasing the ductility and maintaining or increasing the yield stress of the steel; and (c) forming a steel product.
 6. The method defined in claim 5 wherein the mechanical working step (a) includes cold rolling or drawing or any other suitable mechanical working step that reduces the transverse cross-sectional area of the feed steel.
 7. The method defined in claim 5 wherein the mechanical working step (a) includes cold rolling or drawing or any other suitable mechanical working step that changes the cross-sectional shape of the feed steel, without necessarily changing the transverse cross-sectional area, such that there has been energy input required to cause the shape change.
 8. A method of producing a steel product includes: (a) mechanically working a low carbon, medium carbon or high strength low alloy steel product, and (b) heat treating the mechanically worked steel product and increasing the ductility and maintaining or increasing the yield stress of the steel.
 9. The method defined in claim 8 wherein the mechanical working step (a) includes cold rolling or drawing or any other suitable mechanical working step that reduces the transverse cross-sectional area of the steel product.
 10. The method defined in claim 8 wherein the mechanical working step (a) includes cold rolling or drawing or any other suitable mechanical working step that changes the cross-sectional shape of the feed steel, without necessarily changing the cross-sectional area, such that there has been energy input required to cause the shape change.
 11. A method of producing a steel product includes: (a) mechanically working a low carbon, medium carbon or high strength low alloy steel feed steel, (b) forming the steel product from the mechanically worked feed steel, and (c) heat treating the steel product and increasing the ductility and maintaining or increasing the yield stress of the steel product.
 12. The method defined in claim 11 wherein heat treatment step (c) also includes heat treating the formed steel product and maintaining or increasing the tensile strength of the steel product.
 13. The method defined in claim 11 wherein the mechanical working step (a) includes cold rolling or drawing or any other suitable mechanical working step that reduces the transverse cross-sectional area of the feed steel.
 14. The method defined in claim 11 wherein the mechanical working step (a) includes cold rolling or drawing or any other suitable mechanical working step that changes the cross-sectional shape of the feed steel, without necessarily changing the transverse cross-sectional area, such that there has been energy input required to cause the shape change.
 15. A method of producing a steel product that includes selecting a low carbon, medium carbon or high strength low alloy steel feed steel as a starting material for the product and selecting mechanical working and heat treatment time and heat treatment temperature conditions of the feed steel or a product made from the feed steel to provide required mechanical properties for the product and carrying out mechanical working and heat treating steps and maintaining or increasing ductility and maintaining or increasing the yield stress of the steel and producing the product with the required mechanical properties.
 16. A mechanically worked and heat treated low carbon, medium carbon or high strength low alloy steel product made by the method defined in claim
 1. 17. The steel product defined in claim 16 includes any one of wire, rod, bar, or strip.
 18. The steel product defined in claim 17 includes a steel product that is made from any one of wire, rod, bar, and strip. 